PHOTOIONIZATION-EFFICIENCY SPECTRUM AND IONIZATION-ENERGY OF THE CYANOMETHYL RADICAL CH2CN AND PRODUCTS OF THE N(S-4)+C2H3 REACTION

Photoionization efficiency (PIE) spectra of the CH2CN radical were mea sured over the wavelength range lambda = 115-130 nm using a discharge- flow-photoionization mass spectrometer coupled to a dispersed synchrot ron-radiation source. The cyanomethyl radical was produced by the reac tion F + CH3CN --> CH2CN + HF, and the PIE spectrum displayed steplike behavior near threshold, From the half-rise point of the initial step , a value of 10.28(0) +/- 0.01(0) eV was obtained for the adiabatic io nization energy (IE) of CH2CN based on five independent determinations . From a single measurement of the PIE spectrum and threshold for CD2C N, we obtain IE(CD2CN) = 10.24 eV. The experimental result for CH2CN i s compared with previous measurements, estimates, and calculations. Th e present PIMS study of the CH2CN radical provides experimental measur ements of the adiabatic ionization energy that are simultaneously the most direct and the must precise available. For the reaction N(S-4) C2H3, the C2H2N radical product exhibits a PIE spectrum that may inclu de CH2CN along with another species that has a gradual threshold that is at a considerably longer wavelength than the steplike threshold of CH2CN (derived from F + CH3CN). A possible source of this difference i s the contribution from higher-energy C2H2N isomers and/or from excite d CH2CN, In sharp contrast to the results for the N(S-4) + C2H3 reacti on, no signal attributable to an isomer of the C2D2N radical was obser ved from the N(S-4) + C2H3 reaction, The C2H3N/C2D3N adducts from the N(S-4) + C2H3/C2D3 reactions were also studied. The adduct was observe d to be solely CH3CN for the N(S-4) + C2H3 reaction, while for N(S-4) + C2D3, the PIE spectrum appears to include significant contributions from both the lowest-energy isomer CD3CN and one or more higher-energy isomers.